Refactor: Get rid of setup.py

I propose transitioning to using just a `pyproject.toml` file, following [PEP 621](https://peps.python.org/pep-0621/) and [PEP 631](https://peps.python.org/pep-0621/).

For an example of a package that uses `pyproject.toml` exclusively, see [pdf2sents](https://github.com/allenai/paper-details-on-demand/tree/main/pdf2sents), a component for the SCIM feature. As you can see from its [`pyproject.toml`](https://github.com/allenai/paper-details-on-demand/blob/1ed8ab62f1504b049ade3de396478abe3af59c48/pdf2sents/pyproject.toml), all dependencies, package metadata, and even configuration for plug-ins such as `black` and `mypy` are all part of the TOML file.

Installation procedure via `pip` is unchanged.